The quoted message means that backup should have been placed on one extent (as it's dictated by the specified policy), but due to some force majeur, such as absence of free space or something different, it was placed on other extent.

However, it seems that both extents have quite a lot of free space, so, the described behaviour doesn't look expected and should be investigated by support team.

We have the exact same issue on Veeam 9.5 Update 3 and we are planning to solve it with an Active Full backup. Just wondering (if this is a product issue) whether updating Veeam to 9.5 Update 3A would help with the issue or not? (If not, will a future version do?)